The present invention relates to a lithium secondary battery which is capable of suppressing reduction in capacity due to charge/discharge cycles and has an efficient charge/discharge cycle property.
From a point of view of environmental problems, attention has been attracted to the lithium secondary battery as a battery for electric vehicles and a battery to store electric power. For such uses, the battery requires a high energy density and a long life of more than 1000 cycles of repetitive charge and discharge operations. To obtain a high energy density, the lithium secondary battery conventionally employs graphite materials for its negative electrode. This, however, leads to a drawback that the cycle life of the battery becomes shorter.
In operation of the lithium secondary battery, lithium ions reciprocate between a negative electrode and a positive electrode opposing thereto through an electrolyte and a separator. During this, due to decomposition and formation of the electrolyte and/or reaction and formation between the electrolyte and the negative electrode, a highly resistive film of LiF, Li2CO3, LiOH, or the like is generated and deposited on a surface of the negative electrode. As a result, the internal resistance of the lithium secondary battery becomes higher and movement of lithium ions is hindered, leading to a reduction in the charge/discharge capacity. By observing the surface of the negative electrode, it has been confirmed that the negative electrode is covered with the highly resistive layer. Hence it is recognized that prevention of the generation of the continuous highly resistive layer effectively suppresses the reduction in the charge/discharge capacity.
To remove the problem of the reduction in the charge/discharge capacity of the lithium secondary battery, JP-A-2008-159333 describes a technique in which in a nonaqueous lithium secondary battery including a positive electrode and a negative electrode which are capable of intercalating and releasing lithium ions and a porous film which separates the positive electrode from the negative electrode, a ceramic coating layer is disposed on a surface of the negative electrode. Also, JP-A-2005-327680 describes a technique in which inorganic oxide fillers are disposed similarly on a surface of the negative electrode. These techniques will be essential to the improvement of the safety and reliability of the nonaqueous lithium secondary battery. Further, WO97-01870 describes a technique in which a mixed layer including particles of active material and particles of inorganic oxide is formed on a surface of the negative electrode.